Process of chemical nickel plating and bath therefor



Patented Nov. 10, 1953 UNITED STATES PATENT OFFICE PROCESS OF CHEMICAL NICKEL PLATING AND BATH THEREFOR Gregoire Guizeit, Highland, Ind., and Ernest J. Ramirez, Forest Park, Ill., assignors to General American Transportation Corporation, Chicago, 111., a corporation of New York Application January 4, 1951, Serial No. 204,424

as steel, with nickel, by contacting that materialwith an aqueous acid bath containing nickel ions and hypophosphite ions and a buffer in the form of a salt of an organic acid. That process is carried on under certain optimum or critical conditions, hereinafter more specifically mentioned, whereby the process and bath there described have many advantages in commercial use over electroplating methods, including low cost of equipment, good adhesion of the coating in the finished product, and a coating which is con- 'tinuous, uniform, very hard, and possessed of high corrosion resistance.

The optimum conditions under which the process described in said application are carried :on are as follows: The ratio between nickel ions and hypophosphite ions in the bath, expressed in molar concentrations, is within the range of 0.25 to 0.60; the absolute concentration of hypophosphite ions expressed in mole/liter in said bath is within the range of 0.15 to 0.35; the absolute concentration of the buffer is approximately equivalent to two carb-oxyl groups for every nickel ion that can be deposited, for instance, :in the case of sodium acetate, 0.120 mole/liter of acetate ion; the initial pH of the bath is within the approximate range of 4.5 to 5.6; the temperature of the bath is slightly below the boiling point; and the ratio between the volume of the bath, expressed in cubic centimeters and the surface area of the body being plated, expressed in square centimeters, V/A, is not greater than 10. The buffering agents mentioned in said application and employed in the tests therein referred to are soluble acetates, such as sodium acetate.

It has been found that even when the above mentioned most favorable conditions exist, i. e., a ratio of nickel ions to hypophosphite ions between 0.25 and 0.60 and a hypophosphite ion concentration in the bath, expressed in mole/liter between 0015 and 0.35, and when sodium acetate is used as a buffer, the bath is relatively unstable with a tendency to form a black precipitate after the i ha bee oi g n for some 19 Claims. (Cl. 117-130) time, i. e., a random chemical non-catalytic reduction of the nickel ions eventually takes place. High absolute concentrations of hypophosphite ion favor this objectionable decomposition of the bath.

, The present invention involves the discovery that the rate of plating in such a process may be substantially increased and the plating bath stabilized as well to prevent the formation of a black precipitate, even with high V/A ratios and with relatively high hypophosphite ion concentrations, by using as an exaltant in the bath soluble salts of simple short chain aliphatic dicarboxylic acids which are defined as saturated aliphatic acids without other functional groups than 0H having from three to six carbon atoms in their molecule, and an ionization constant (pKaz) over 5.0. Their general formula is boxylic acids when used as an exaltant in chemical nickel plating baths, is a direct function of their ionization constant and of the number of carbon atoms in their molecule, as hereinafter demonstrated.

Chemical nickel plating, and the plating deposition of other catalytic metals, in an acid bath, is based upon the catalytic reduction of metallic ions, such as nickel ions, by hypophosphite ions in the presence of water, according to the general equation:

- Surf.

2' 2NaH(HP 03) Ni 21101 H, or expressed in ionic form:

Soluble hypophosphites dissolved in Water, i. e.,

rates of nickel plating in a bath containing hypophosphite ions, are oxidized to the corresponding phosphites in the presence of a catalytic surface such as steel or nickel even in the absence of metal ions, with the evolution of hydrogen at the catalytic surface according to the following equation:

(3) Catal. 7 Na(HzP 02) H2O ET N The rate of hydrogen evolution is a function .of the catalytic area and of the temperature. It has been found that under conditions which are otherwise identical, this rate can be substantially increased by the addition of organic carboxylic ions.

It is postulated that the ions of these organic acids form heteropolyacid anions with the hypophospite ion and that the reducing power of these complex anions is .much greater than that of the simple hypophosphite, a well known phenomenon called exaltation (or activation) by complex formation. Whatever the reason for the increase in the rate of catalytic oxidation of the hypop'hosphite ion to phosphite ion may be, it can be measured experimentally by determining the amount of hydrogen evolved per unit of time with and without the addition of the exalting chemical, with all other conditions remaining equal.

It has also been found "that the corresponding e proper amounts of nickel ions and hypophosphite ions, at a s table initial pH, are a function of the rate of hypophosphite oxidation as meas-- ured by the hydrogen evolution.

If per cent exaltation is arbitrarily defined as the per cent increase in the rate of hydrogen evolution with reference to a given bath, it has been found that the soluble salts of simple short chain aliphatic dicarboxylic acids of the general formula R l-430E where R stands for an aliphatic radical having from one to four CH2 groups, will result in a high degree of exaltation, as shown by the tests hereinafter referred to. Also, the use of this addition material retards considerably the formation of the black precipitate and prevents its formation for all practical purposes within the time of plating.

It is, therefore, the primary object of the present invention to provide an improved chemical nickel plating process of the character above described in which the reaction involved is carried out more efliciently and more perfectly than theretofore, thereby rendering the process more desirable from a commercial standpoint.

A further object is to provide an improved aqueous bath which may be employed with advantage in the practice of the improved process.

Still another object of the invention is to provide an improved process of nickel plating by a chemical bath of the character described which includes as an exaltant a soluble salt of a simple short chain aliphatic dicarboxylic acid.

These and other objects and advantages of the present invention will be understood from the foregoing and the following description taken with the accompanying drawings, in which,

Figure 1 shows the variations in percentage of exaltation with changes in total carbon atoms in the aliphatic chain when using additive in the monocarboxylic and the dicarboxylic groups;

Fig. 2 shows the variations in the percentage of exaltation with changes in the ionization constant when using additives in the monocarboxylic and dicarboxylic groups;

Fig. 3 shows the variation in the weight of the plating deposited with changes in the ratio of nickel ions to hypophosphite ions with a succinate additive;

Fig. 4 shows the variation in the weight of the plating deposited with changes in the initial pH of the bath when using sodium succinate as an :exaltant;

Fig. 5 shows the variation in the weight of the plating deposited with changes in the hypophosphite ion concentration with succinate ion concentrations of three different values; and

Fig. 6 shows the variation of the weight of a plating deposited with changes in hypophosphite ion concentration through a wide range when using a relatively low succinate ion concentration as an additive.

In accordance with the process of the present invention, the catalytic material that may be coated with nickel comprises any material in the solid phase which will initiate at its surface the reaction of Equations 1 and 2 set forth above; i. e., a material which, when immersed in the bath, will cause the evolution of hydrogen gas at its own surface. The following elements are examples of catalytic materials which may be nickel plated: copper, silver, gold, beryllium, boron, germanium, aluminum, thallium, silicon, carbon, vanadium, molybdenum tungsten, chromium, selenium, 'tellurium, titanium, iron, cobalt, nickel, palladium and platinum; and the following elements are examples of noncatalytic materials which ordinarily may not be nickel plated: bismuth, cadmium, tin, lead and maganese. The activity of these catalytic materials varies considerably. The following elements are particularly good catalysts in the chemical nickel plating bath hereinafter 'set forth, viz: aluminum, carbon, chromium, cobalt, iron, nickel and palladium.

The nickel plating process becomes autooatalytic, when both the original surface and the metal that is deposited are catalytic, and the reduction of the nickel ions to metallic nickel in the bath in accordance with the reaction of Equation 2 proceeds until all of the nickel ions have been reduced to metallic nickel, in the presence of an excess of hyp'ophosphite ions, or until all of the hypophosphite ions have been oxidized to phosphite, in the presence of an excess of nickel ions. Actually the reaction of Equations 1 and 2 is slowed down rather rapidly as time elapses, because the anions, as contrasted with the cations, of the nickel salt combine with the hydrogen cations to form an acid, which, in turn, lowers the pH of the bath tending to dissolve the nickel deposit, in accordance with equation Also, the reducing power of the hypophosphite ion is decreased as the pH value decreases. It is, therefore, important to prevent a rapid drop of the pH of the bath after initial adjustment thereof within its optimum range. This can be achieved with some degree of success by various schemes used either alone or in combination. For example, the pH can be kept within the optimum range by continuous or periodic addition of an alkali, preferably weak, such as sodium bicarbonate. However, according to the present invention, an exaltant is supplied in the form of a soluble salt of a simple short chain aliphatic dicarboxylic acid, thereby obtaining the advantages of a high plating rate and an absence of the black precipitate heretofore mentioned. It so happens that this exaltant acts as a buffer if buffering is necessary.

For the purpose of studying the exaltation of the hydrogen evolution with the use of different additives including those involved in the discovery of the present invention, 1. e., salts of simple short chain dicarboxylic acids, and other exalting agents which have been used or suggested for chemical nickel plating, tests have been made with baths consisting of 50 cc. of sodium hypophosphite having an absolute anion concentration of 0.225 mole/liter [to which a trace of a nickel salt (0.0024 mole/liter) has to be added to initiate rapidly the reaction] with a pickled piece of mild steel, cm. in area, in the bath, at a temperature of 99 C. for periods of minutes each, using as additives, first, a series of saturated monocarboxylic acid salts: formic, acetic, propionic, butyric and valeric having, respectively, 1, 2, 3, 4 and 5 carbon atoms in the aliphatic chain and, second, a series of saturated dicarboxylic acid salts: malonic, succinic, glutaric, and adipic having, respectively, 3, 4, 5 and 6 carbon atoms in the aliphatic chain. The hydrogen evolved was collected and its volume measured by usual methods. The additives (when used) were employed as the sodium salts of the acids in a concentration of 0.125 mole/liter of the anion.

The results with monocarboxylic acid salts as additives, as to volume of hydrogen evolved, ionization. constants and percentages of exaltation are shown by the following table:

These results, as to exaltation with variation in carbon atoms are also shown by the curve 2!.) in Fig. 1 of the accompanying drawings where the number of carbon atoms has been plotted as abscissae and the percentages of exaltation have been plotted as ordinates. In these tests the best exaltation effects were obtained with ionization constants of 4.75 or higher, as shown by the curve 22 in Fig. 2 of the drawings where the ionization constants are plotted as abscissae and the percentages of exaltation as ordinates, and the maximum effect was obtained with the propionic ion. However, the higher monocarboxylic fatty acids (having 3 or more carbon atoms) have a very disagreeable odor which renders them unsuitable from the practical standpoint. Their solubility in water at a pH of around 5.0 is also very low.

These tests were repeated with an unsaturated ion corresponding to the propionic anion, namely, acrylic, with the result that a negative exaltation effect of minus 50% was obtained, that is, the volume of hydrogen gas evolved was a 6 only 40 cc. as compared with cc. without any additive.

Upon repeating these tests with the above mentioned saturated simple short chain aliphatic dicarboxylic acids as the exalting additives, the volume of hydrogen gas evolved (corresponding to an increased plating rate in the presence of nickel ions) was much greater, as shown by the curve 24 in Fig. 1 of the accompanying drawings and by the following table:

Table II I70k of C'Iogal I Ionizay roar on No.0 tion g g gg gen Atoms CH1 Cong i Anion) Evolved in Ali- Groups stant in 30 phatic (R) pKaz MinS. Chain (25 C In'these tests the best exaltation effects were obtained with acids having an ionization constant of their second hydrogen above 5.6, as shown by the curve 26 in Fig. 2.

The chemical formulae of the dicarboxylic acids mentioned in Table II are as follows:

Malonic acid:

COOH

OOH

Succinic acid:

0 O O H HxC O O H Glutaric acid:

C 0 O H O O H Adipic acid:

COOH

(BOOB:

succinate are commercial chemicals. Adipic acid,

while commercially available, has a comparatively low exaltation effect.

Similar tests under the same conditions with non-saturated dicarboxylic acids show no exalting eflect. For example, maleic acid. which has an unsaturated radical corresponding to succinic acid, as shown by its formula:

does not cause any increase in the volume of hydrogen evolved.

In carrying out the process, the article to be plated and normally formed of the catalytic material is properly prepared by mechanically cleaning, degreasing and light pickling, according to the standard practice in electroplating processes. For example, in the nickel plating of a steel object, it is customary mechanically to clean the rust and mill scale from the object, to degrease the object, and then lightly to pickle the object in a suitable acid, such as HCl. The article is then immersed in a suitable volume of the bath containing the proper proportions of nickel ions, hypophosphite ions and an exaltant, the pH of the bath having been, if necessary, adjusted to an optimum value by the addition of a suitable acid, and thebath having been heated to a temperature just below its boiling point, such as 99 centigrade at atmospheric pressure. Almost immediately, hydrogen bubbles can be observed forming on the catalytic surface of the steel object and escaping in a steady stream from the bath, while the surface of the steel object is slowly coated with nickel (containing some phosphorus). The reaction is continued until the color of the bath (green at the start) shows the absence of nickel, or until the evolution of hydrogen gas stops.

As indicated above. a nickel plating bath containing a soluble nickel salt, a soluble hypophosphite, and a buffer in the form of a soluble salt of a monocarboxylic acid, such as sodium acetate, is only relatively stable so that, even without the presence therein of a catalytic surface, it tends to decompose more or less rapidly by a random chemical reduction of the nickel ions. Specifically, the nickel ions are reduced as a fine amorphous, black powder, which, in turn, acts as a catalyst. The resulting precipitate is gray to black and contains various quantities of nickel, phosphorus and salts, depending on the conditions of formation, thus giving rise to the term black precipitate. This spontaneous decomposition is a function of temperature, time and initial bath composition and, insofar as the initial bath composition is concerned, the higher the ratio of hypophosphite ions to nickel ions and the higher the absolute concentration of hypophosphite ions, the more unstable is the bath. Thus, instead of rendering the bath capable of controlled nickel deposition in the presence of a catalyst, a high concentration of hypophosphite anions will produce the well known purely chemical nonselective reduction of nickel ions. Under plating conditions, in the presence of a catalytic surface and at an elevated temperature, the tendency to form this black precipitate is even more pronounced. The formation of the black precipitate in the bath is highly objectionable in that the presence of the conditions causing the black precipitate in the bath result in a rapid decomposition of the bath by the uncontrolled re duction of the nickel ions in the bath, thus depleting the bath. Also, the presence of the black precipitate in the bath results in a dull, rough and uneven nickel coating upon the object which is undergoing the plating reaction. However, when an exaltant is used in the form of a soluble salt of a simple short chain aliphatic dicarboxylic acid, as described above, very little, if any, black precipitate is formed and, at the same time, there is gained the great advantage of a highly increased plating rate.

In carrying out the process of the present invention according to the procedure which has been outlined, it is preferable to keep within the optimum ranges as to temperature, concentrations, ratios and the like which have been summarized above as the conditions under which the process described in said co-pending application is carried out, although the present invention permits a somewhat broader scope, as will now more fully appear.

Accordingly, it is important that the temperature of the bath be maintained at the highest possible degree below the boiling point under the prevailing conditions, that is, approximately 99 C. under standard conditions, for the reason that the rate of catalytic reduction of nickel ions to metallic nickel is a function of the temperature of the bath, and this function is logarithmic, so that the plating rate tends to drop very rapidly with a drop in temperature.

Further, though there is a definite relation in the plating rate of the object in the bath between the volume of the bath and the surface area of the object, in addition it has been found that the tendency to the formation of a black precipitate increases with an increase in the volume of the bath. The best values of this ratio (V/A) between the volume of the bath (cmfi) and the geometric surface area (cm?) of the object being plated are below 10.

With respect to the composition of the bath, it essentially comprises an aqueous solution containing nickel ions, hypophosphite ions and an exaltant and may be formed by employing a soluble nickel salt, a soluble hypophosphite and an exaltant in the form of a soluble salt of a dicarboxylic acid of the character referred to above. For example, the nickel ions may be derived from nickel chloride or nickel succinate and the hypophosphite ions may be derived from sodium, potassium, lithium, calcium, magnesium, strontium, barium, etc., hypophosphites or various combinations thereof. It has been found that certain alkaline cations that may be thus introduced into the bath appear to retard the rate of nickel deposition with respect to other cations. For example, barium ions appear to retard the rate of nickel deposition with respect to sodium and potassium ions.

In preparing the bath, the amounts of soluble nickel salt and soluble hypophosphite that are employed are such that both the ratio of nickel ions to hypophosphite ions and the absolute concentration of hypophosphite ions are initially established within optimum ranges. The term ion as employed herein includes the total quantity of element or radical present in the bath; i. e., both undissociated and dissociated material. In other words, dissociation is assumed when the term ion is used in connection with molar ratios and concentrations in the bath. The ratio between the nickel ions and the hypophosphite ions, Ni++/ (H2PO2) in terms of molar concentrations, may be expressed as a decimal fraction; and it has been discovered that the most favorable range of this fraction lies between 0.25 and 1.60 with a definite optimum between 030 and 0.80. It has also been found that the required absolute concentration of the exaltant ions, as in the case of a buffer, should be roughly equivalent to at least two carboxylic groups per ion of nickel that can be reduced, as explained more fully hereinafter. For example, if a hypophosphite ion concentration of 0.225 mole/liter is chosen, the required absolute concentration of dicarboxylic exaltant anions, expressed in mole/liter, would be approximately 0.06. The optimum absolute concentration of hypophosphite ions in the bath, i. e., one that will result in good nickel plating without an excessive tendency to spontaneous decomposition is within the range of 0.15 mole/liter to 1.20 mole/liter.

The increased plating rate brought about by the use as an exaltant of soluble salts of sat urated simple short chain dicarboxylic acids, according to the present invention, is shown by the following comparative tests. A 50 cc. plating bath containing 0.122 mole/liter of hypophosphite ions (1.9% calcium hypophosphite), 0.115 mole/liter of acetate ions, and 0.075 mole/liter of nickel ions, adjusted to an initial pH of 5.1 with hydrochloric acid, at a temperature of 99 C., deposited 0.0732 gram of nickel coating in 10 minutes on a mild steel sample of 20 cm. area (low V/A ratio). 100 cc. of the same plating bath and under the same conditions deposited 0.1932 gm. of nickel coating in 2 hours on a mild steel sample of 5 cm. area (high V/A ratio). In contrast with these results, a 50 cc. plating bath containing 0.122 mole/liter of hypophosphite ions (1.9% calcium hypophosphite), 0.075 mole/liter of succinate ions, and 0.077 mole/liter of nickel ions with no pH adjustment, at a temperature of 99 C., deposited 0.1232 gm. of nickel coating in minutes on a mild steel sample of 20 cm. area (low V/A ratio). 100 cc. of the same plating bath and under the same conditions, using succinate ions, deposited 0.3132 gm. of nickel coating in 2 hours on a mild steel sample of 5 cm. area (high V/A ratio).

In these tests, the difference in the weights of the coatings is 0.05 gm. for 10 minutes with the low V/A ratio and 0.12 gm. for 2 hours at the high V/A ratio. This represents an increase in rate of about 68.3% and 62.11%, respectively. Moreover, the quality of the plating was much brighter and smoother when succinate was used as an exaltant.

In further comparative tests, using acetate ions and succinate ions, a cc. optimum plating bath, containing 0.224 mole/liter of hypophosphite ions, 0.120 mole/liter of acetate ions, and 0.08 mole/liter of nickel ions, adjusted to a pH value of 5.01, at a temperature of 99 C., deposited 0.089 gm. of nickel coating in 10 minutes on a mild steel sample of 20 cm. area. The plating was of fair quality.

The test just referred to was repeated with a 50 cc. plating bath containing 0.224 mole/liter of hypophosphite ions, 0.055 mole/liter of succinate ions, and 0.0765 mole/liter of nickel ions, at a temperature of 99 C., and this bath deposited 0.1180 gm. of nickel coating on a mild steel sample of 20 cm. area in 10 minutes. No pH adlustment was used and the plating was very bright and smooth. The difference in the weight of the two coatings obtained in these two tests, with practically identical bath compositions except for the difierence in the buffer, is 0.029 gm., an increase of about 30%. When plating with succinate ions as an exaltant, the quality of the plating was far superior to that obtained when using acetate ions. In general, even better results are obtained by the addition of larger amounts of sodium succinate.

The optimum range of the ratio of nickel ions to hypophosphite ions is preferably as set forth in the copending application, namely, between 0.25 and 0.60 but the ratio appears to be less critical when succinate ions are employed and considerably broadened in scope, as is shown by the curve 28 in Fig. 3 of the accompanying drawings which shows the results of a plating test with a V/A ratio of 100 cm. to 5 cm. on a steel sample with 0.063 mole/liter of succinate ions at an ini-. tial pH of 5.5. The ratios of nickel ions to hypophosphite ions are plotted as abscissae and the Weights of the plating deposited in milligrams per cm. in a period of 2 hours are plotted as ordinates. In the tests represented by the curve 28 in Fig. 3, a black precipitate formed after one hour when the ratio of nickel ions to hypophosphite ions was below 0.2 and after minutes when the ratio was about 0.250 but for all higher ratios no black precipitate was formed; however, the plating was satisfactory over the entire range of ratios.

It has been found that the amounts of exalting additive used when dicarboxylic acid salts, such as sodium succinate, are employed is about half of that required when monocarboxylic acid salts, such as sodium acetate, are employed as a bufler. It is also true that no pH adjustment is required when using sodium succinate as an additive. The bath can be used as prepared with its pH between 5.8 and 6.5. In that case there will be some green precipitate formation but it does not interfere with the plating operation. Also, baths prepared with succinate as an exalting additive are far less sensitive to initial pH variations which is a decided advantage in chemical plating.

The effect of variation in the initial pi-l value upon the weight of nickel deposited is shown by a further test, the results of which have been plotted to form the curve 30 in Fig. 4 where the initial pH values are plotted as abscissae and the weight of nickel deposited in milligrams per cm. in a period of 2 hours are plotted as ordinates. In this test, the plating was carried on at a high V/A ratio (a steel sample of 5 cm. area in cc. of solution) at a temperature of 99 C., with sodium succinate as the additive. The bath contained 0.283 mole/liter of hypophosphite ions, 0.094 mole/liter of nickel ions and 0.055 mole/liter of succinate ions. The ratio of nickel ions to hypophosphite ions in the bath was 0.333. The initial pH values were adjusted with the values indicated on the curve by hydrochloric acid except that a pH value of 6.25 was obtained with the use of sodium bicarbonate (NaHCOs). The plating was bright and smooth with pH values of 5.0 and below but a yellow-green deposit appeared with higher pH values.

An important feature about plating with baths containing dicarboxylic anions, such as the succinate ion, is the fact that the formation of black precipitate is practically eliminated when operating under the standard optimum operating conditions which have been prescribed above and which are set forth in said copending application. The plating reaction is well controlled and no random chemical reduction of nickel occurs. To

demonstrate this fact, a plating bath having a ratio of nickel ions to hypophosphite ions of 0.333 and an absolute hypophosphite ion concentration of 0.225 mole/liter, with 0.05 mole/liter of succinate ions present, was heated to 99 C. for 8. hours and no trace of black precipitate appeared.

It is postulated that this result. arises from the fact. that. the hyp p p ite succinate heter polyacid ion, responsible for. the exaltation effect, forms a relatively insoluble basic. nickel com- Pound, probably a c mplex, which is in. equ lib rium with the solution. By measuring he. volt,- agc change in a cell having one nickel lectrod s one calomel electrode, and a 0.003 molar nickel chloride solution as the electrolyte, before and after the addition of 0.25. mole of; sodium succinate and 0.25 of sodium hypophosphite,. it has been found that 70% of the nickel was tied up. Neither hypophosphite alone nor succinate, alone, under identical conditions. showed any nickel complex formation... The. appearance of a pale green precipitate can actually be observed durin the. plating operations. carried. out at 99 C... at pH valu s. above 4-8:. An analysis of. this precipitate showed that it contained 15.11% of. nickel,.13;.3% of phosphorus and 11.94% of succinate. radical. This. precipitate can. be dissolved y reducing. the pH. of the. value of the bath below 5.0 which decreases somewhat the rate of plating but. othen wise hasno ill efiects.

It. has b en. stated above and insaid. copendin application that the. amountoi additive as buffer or ex ltant should correspond to. the amount of. ruckcl ion that canbe. reduced by the. am untof QD -P o Pres nt. in. the. ba h. In. other words, the concentration of the additive sho ld be a eq ate to take car of he hydrogen ions liberated by the reduction of the nickel ions to metal c n kel- Th s. corre ponds. to 2 carb l ro p r mo of. reduced ni l ion o i the s Of. 3t mo ar xvl c 4 m al 51 6 as sodium acetate, to 2 moles of acetic anion per moleof nickel ion. The same ratio exists in the case of dicarboxylic acids and, 1. mole of the anion or 2 carboxyl groups is. therefore neededfor each mole of reduced nickel ion.

A large excess of; dicarboxylic ions, such. as succinate ions, above the nickel concentration will, tie up" all of the available cation, as a complepnso that no plating will; take place, If thenickelion concentration is further increased, while keeping the amount of dicarboxylic ion constant, there will form on the catalytic surface. a dull gray nonmetallic coating. which may be described as. crusty. Upon increasing still further the nickel concentration, good nickel plating willv start. For example, using a, ratio of nickel ions to hypophosphite ions of 0.333, a constant amount or ;05- mole/liter of succinatc ions and increasing the absolute concentration of-the hypophosphite ions, with a corresponding increase of the nickel ions to keep'theratio; at 0.333, no plating whatsoever occurred at a high V/A ratio, with a bath temperature of 999 C. in a period of 2 hours untilthe hypophosphite ion concentrationwas brought up to avalue ofabout 0.06 mole/liter. From that point. until thehypophosphite ion concentration reached 0.10 mole/liter a dull gray crust was formed on the steel sample. A. bright metallic coating; started to buildup at a minimum hypophosphite concentration of 0.114 mole/liter. For, practical purposes, it may be stated that the ratio of hypophosphite-ions tosuccinate ions should be better than 2 to 1 in order to obtain thebest results.

The effects of increasing the concentration of the hypophosphite ions at three different levels of they succinate ion concentration are shown by the curves 32, 34 and 36in Fig. 5. oi, the accompanying drawings. which have been obtained by plotting the data obtained in tests run with pH values as normally prepared without adjustment and with the ratio of nickel ions to hypophosphite ions maintained constant at 0.333. The volume of the solution was 50 cc. and the tests were run for 2. hours while plating a 12- gauge steel sample of 20 cm. area. The hypophosphite ion concentrations in mole/liter were plotted as abscissae and the weights of the plating in the 2 hour periods, in milligrams per cm}, are plotted as ordinates. The three curves 32.. 34 and 35 show the results with succinate ion concentrations of 0.037, 0.074 and 0.111 mole/liter, respectively.

As heretofore. stated, when a soluble acetate is used as a buffer in chemical nickel plating, the absolute concentration of the hypophosphite ions is critical, being optimum above. 0.15 mole/liter and below 0.35: mole/liter. If the hypophosphite concentration is further increased, the black precipitate appears and consumes the available nickel ions, thus interfering with the plating. Under similar conditions, but using the succinate ions in. an amount of about 0.05 mole/liter, which is roughly corresponding, it has been found that the hypophosphite ion concentration can be increased upto 1.2 mole/liter before the random reduction of the nickel ions occurs.

This is shown by the results obtained tests during which a lit-gauge steel sample oi 5 cm area was. plated in a. cc. bath containing. 01.5- mole/liter of succinate ion. during a period of 2 hours. at 99 C. The ratio of nickelv ions to hypophosphite ionswas kept at 0.333 and the pH value. wasthat obtained by preparation of the solution without adjustment. The results of this test, up to hypophosphite ion concentrations. 01 1.20mole/liter, are shown by the curve 38 in Fig. 6. of. the. accompanying drawings where the hypophosphite ion concentrations in moles/liter are plotted as abscissae and; the weight of the. plating obtained. in. the. 2-hour. period, in. milligrams per cm. are plotted as ordinates. For all hypophosphite; ion concentrations. from 0.114 to 1.20 ,mole/liten. the coating was bright and smooth. In these tests, at. an. absolute hypo phosphite-concentration;of 1.4.moles/liter. a black precipitate, appeared. after 1 hour. and30 minutes and the weight of the plating deposited. was 0.243 gmi. At. an. absolute hypophosphite concentration. of 2.0 moles/liter,. the black. precipie tate started forming: almost. immediately and the. plating. could; be. carried on only for: aperiod of 30 minutes: with a resulting nickel deposit: of 0.115. gm. Thistest shows-that with. the. use. of snccinate ion. as an. exaltant, the. optimum .range of. plating. conditions. can. be. broadened substane tially without the disadvantage-of. the blaclrprecipitate formation;

In. plating. according to. the presentinvention, the succinate. ion is-.not.used-.up inthe plating, operation but forms slightly dissociated succinic acid. so: that. an exhausted. plating. solution. can be. regeneratedby readjusting. the-nickelion and the hypophosphite.ionconcentration-.andby neurtralizing the succinic. acid-to the. proper. pH value by means of a mild; agent, such assodium bicarbonate... This technique. has; the. advantage that a brighter coating may thereby be produced From the foregoing. it is apparent that great advantages are obtained by mouse ofan exaltant in the form of a simple'short chain aliphatic. diecarboxylic acid. It1Will be. understood. that the invention is.not limited. except. asdefinedby the appended claims.

13 We claim: 1. The process of chemically plating with nickel a catalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a salt of a simple short chain saturated aliphatic dicarboxylic acid. 2. The process of chemically plating with nickel a catalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a salt of a saturated dibasic lower aliphatic acid the molecule of which comprises from three to six carbon atoms.

3. The process of chemically plating with nickel a catalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprise contacting said material with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a salt of a saturated aliphatic dicarboxylic acid of the general formula o C/ H R on where R is a radical containing from one to four =CH2 groups.

4. The process of chemically plating with nickel a catalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a saturated aliphatic dicarboxylic acid salt of the general formula where R is an alkyl radical comprising from one to four carbon atoms and M is an alkaline metal. 5. The process of chemically plating with nickel a catalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a saturated aliphatic dicarboxylic acid salt of the general formula ONa 0 14 whereR is an alkyl radical comprising from one to four carbon atoms.

6. The process ofchemically plating with nickel acatalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with an aqueous bath consisting essentially of nickel ions and hypophosphite ions and ions of the general formula where R is a radical containing from two to four: CH2 groups.

8. The process of chemically plating with nickel. a catalytic material essentially comprising an; element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises; contacting said material with a bath consisting; essentially of an aqueous solution of a nickel salt and a hypophosphite and a salt of a saturated aliphatic dicarboxylic acid, wherein the ratio between nickel ions and hypophosphite ions in said bath expressed in molar concentrations is within the range between 0.25 and 1.60, and wherein the absolute concentration of hypophosphite ions in said bath expressed in mole/liter is within the range between 0.15 and 1.20.

9. The process of chemically plating with nickel a catalytic material essentially comprising an element selected from the group consisting of copper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with an aqueous bath consisting essentially of nickel ions and hypophosphite ions and ions of a saturated aliphatic dicarboxylic acid additive, wherein the ratio between nickel ions and hypophosphite ions in said bath expressed in molar concentrations is within the range between 0.25 and 1.60, the absolute concentration of hypophosphite ions in said bath expressed in mole/liter is Within the range between 0.15 and 1.20, and wherein the absolute ion concentration of the saturated aliphatic dicarboxylic acid additive in said bath is at least two carboxyl groups for every nickel ion that can be deposited.

10. The process of chemically plating with nickel of catalytic material essentially comprising an element selected from the group consisting or copper, silver, gold; aluminum, iron, await; nickel, palladium and platinum; winch comprises contacting said material with a bath consisting essentiaiiyoran' aqueoossohition of a nickel salt and a hypophosphite ands sait'orsuccinm acid; whereinthe ratiobetween rfickelf io'ri's andhypm' phosphite ions said hath expressed in molar concentrations is between 0.25 and-1.65 wherein the absolute concentration ofhyipophosphite ions in said hath expressed in mole/"liter is between 0.15 and 1.20, and wherein the absolute concentration of succinate ions in said bath expressed in mole/liter is at least 0.05.

11. The process set forth in claim 8, wherein the initial pH of said bath'is' in the approximate range between 4.3 and 6.8!

12. The process set forth in claim 8, wherein the temperature of said bath is slightly below the boiling point thereof. 2

13. The process of chemically plating with nickela catalytic material essentially comprising element selected from the group consisting or opper, silver, gold, aluminum, iron, cobalt, nickel, palladium and platinum, which comprises contacting said material with an aqueous hath consisting essentially of nickel ions and hypophosphite ions and ion's' of a saturated aliphatic dicarboxylic acid additive, wherein the ratio between nickel ions and hypophosphite ions in said bath expressed in molar concentrations is within the range between 0.25 and-1260, wherein the absolute concentration of hypbphosphite ions in said bath expressed in rnoleVliter is within the range between 0.15 and 1.20, and wherein the ratio between-the mionic or hatnexpressed in cm. and the surface area of said mater-talepresseain cm? isfibt gfatei 111. 7

14-5 sham forthe' chemical plating'cf a catalyric material ixfii th nickel consisting essentiall y or an" aqueous solution of a nickel saltan'd a hypophcsphite and asst: of a'sihipie short chain saturated aliphatic-dicarbeirylic acid.

15. A bath for the chemical plating or-acatalyti'c material vvithmickei consisting essentially Of I anacueous solution *{If 3 a nickel- Salt and a hypophosphite and a salt "or a simpleshort chain saturated aliphatic kiicarboxylic --ac id; I wherein the ratio between hiekel ions ahd hypophos'phite ions in said-hath expressed m molar eoncentra ti'ons Jo'etween -0325 and I 1.60, and wherein the 16 absolute concentration of hypophosphiteibnsin said bath expressed in mole/liter is within the range between 0.15 and 1.20.

16.-The' bath set forth in claim 14; wherein the: initial pH thereof within the approximate range between 4.3' and 6.8. r

l7.-A- bath for the chemical plating oi a datalyt'ic material with nickel consisting essentially of an adileo'ussolution of a nickel salt and a hypophosphite and a salt of a simple short chain saturated aliphatic dicarboxylic acid, wherein the ratiobetwe'e'n nickel ions and hypophosphite ions m'sam' bath ex ressed in molar conc entrations isbetween 0.25 and L60, wherein the absolute concentration'of hypophosphite'ions in said. bath expressed in mole/liter" is within the range between 0.15 and L20, and wherein the absolute concentration of dicarboxylic ions in said bath is atleas't two carboxyl'groups for every nickel ion thatcanbe deposited;

l8; .A'bath for the chemical plating'o-f a catalytic'n'iaterial with nickel consisting essentially of an aqueous solution of a nickel salt, an alkaline. hypopho'sphite and a's'al't of a simple short chain saturated aliphatic dicarbo'ir'ylic acid, wherein the ratio b'etwee'hniekel ions and hypophosphite' ions in' said hath expressed in molar concentraa tio'nsisbetween 0.25 and 1.60, wherein the abso lute concentration of hyp'opho'sphi'te' ions in said bath expressed in mole/liter is within the range between 0.15 and 1.20, wherein the absolute concentration of dicarboxylic'ions in said bath is at least two carboxyl groups for every nickel ion thatcan be deposited, and wherein the initial pH of said bath is within the range between 4.3 and 6.8.

19. A bath for the chemical plating of a catalytic material consisting essentially of an aqueous solution of a nickel salt and an alkaline hypophosphite and an alkaline succinate.

GREGOIRE YGUTZEIT. ERNEST J. RAMIREZ.

References "Cited in the 'fil'e of this patent UNITED STATES PATENTS Nlunrher Name Date 1,207,218- Roux Dec. 5, l9i=fi 2,430,581 Pessel Nov. 11, 1947 2,532,283 Brenner Dec. 5, 1950 

1. THE PROCESS OF CHEMICALLY PLATING WITH NICKEL A CATALYTIC MATERIAL ESSENTIALLY COMPRISING AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF COPPER, SILVER, GOLD, ALUMINUM, IRON, COBALT, NICKEL, PALLADIUM AND PLATINUM, WHICH COMPRISES CONTACTING SAID MATERIAL WITH A BATH CONSISTING ESSENTIALLYI OF AN AQUEOUS SOLUTION OF A NICKEL SALT AND A HYPOPHOSPHITE AND A SALT OF A SIMPLE SHORT CHAIN SATURATED ALIPHATIC DICARBOXYLIC ACID. 